1. Field of the Invention
The present invention relates to an illumination optical system and a projection-type display apparatus.
2. Description of the Related Art
A light source in a conventional liquid crystal projector makes parallel, by a parabolic reflector, light from a high-pressure mercury lamp in which a lamp tube extends in parallel to an optical axis, and emits the parallel light. However, a part of the light reflected by the reflector is blocked by the lamp, and the light utilization efficiency reduces. Accordingly, it has recently been proposed to use a light source with having a high diffusion power, such as a light emitting diode or a laser, or to dispose a lamp tube of a high-pressure mercury lamp perpendicular to the optical axis. It is thus important for an illumination optical system to effectively take in light from the light source with a high numerical aperture (“NA”).
Japanese Patent Laid-Open No. (“JP”) 2005-208571 proposes to collimate a diffused light flux taken from a light source by an aspherical lens. Further, JP 05-273644 proposes to condense a light flux taken from the light source by two Fresnel lenses.
It is important for a projection-type display apparatus, such as a liquid crystal projector, to have a small size with an improved light utilization efficiency. When the light flux from the light source is collimated by an aspherical lens as disclosed in JP 2005-208571, it is difficult to miniaturize the optical system in the subsequent stages. The technique disclosed in JP 05-273644 is advantageous in miniaturization in that the light flux is focused but taking in light from a light source having a high diffusing power requires a considerably large Fresnel, preventing the miniaturization. A tandem system configured to make parallel a flux (in a paraxial region) as disclosed in JP 2005-208571 is more advantageous in enhancing the light utilization efficiency than the focusing system disclosed in JP 05-273644.
For example, assume a liquid crystal projector that includes a pair of fly-eye lenses and a polarization converter array disposed just behind it. The polarization converter array includes micro polarization beam splitters arrayed at regular intervals, and a half waveplate stuck at every other pitch. The polarization converter array converts the non-polarized light flux from the light source into linearly polarized light in one direction. The polarization converter cannot provide a proper polarization conversion and its light utilization efficiency reduces, unless a light source image formed near a second fly-eye lens when it is viewed from the light source side is taken in an effective light area of every other pitch. In the second fly-eye lens, each lens cell thereof corresponds to each lens cell of the first fly-eye lens on a one-to-one basis, and thus the light source image shifted from the corresponding lens cells causes a loss of light. Accordingly, making the light source image formed near the second fly-eye lens a smaller spot improves the light utilization efficiency.
Assume that each of the optical systems disclosed in JPs 2005-208571 and 05-273644 is provided in front of the first fly-eye lens is assumed. Then, the method of JP 2005-208571 causes the later stage of the first fly-eye lens to be bigger in accordance with the angles of the light flux taken in from the light source. For example, the optical system that has an angle of 65° of the light flux taken from the light source becomes bigger by approximately 30% than that has the angle of 45°. Further, in the method of JP 05-273644, the image-side principal point of composite lenses of the Fresnel lens and the first fly-eye lens shifts closer to the light source side than that of the first fly-eye lens alone, causing a large paraxial magnification, a large light source image, and a reduced light utilization efficiency.